The bottom picture provides an example of what I've suggested a couple of times in this thread and elsewhere. In the middle of the top of that map there is what could be a single tile island. 1 hex to it's right is a 2 tile island. 2 hexes to it's left is the coast of the mainland.
Now, I can't quite count the total amount of accessible mainland tiles from that single-tile island, but with the 2 tile island to the right I'm coming up with something close to about 6 hexes in your 36 tiles circumference.
With ample sea resources in that 36 tile area with the single-tile island as it's center, that would give you the potential to construct 7 districts total provided all of the growth came from the sea, and no resources/specific terrain hindered the production of your districts. Considering with land requirements that hit's the "about half" mark, you wouldn't really need to fill all 6 tiles with districts. That leaves room for resource improvements, or even a wonder. The city could still support as much as 4 districts including a harbor.
Now, I think it's a rare find to get
any coastal cit that's better than an fully expanded 36 tile land city simply because the sea is capped at a certain output (a mere 2 food without resources) and can't really be improved upon. That said, I think people are blowing the "uselessness" of islands in civ6 out of proportion nonetheless. Using the little information we know, that 1 tile island with a 2 tile island and continental coast nearby could, in theory, totally become a viable city.
I currently see no evidence that suggest island cities are inherently less useful than they've ever been, really. You only need 1 to tile to build a district. The rest is all growth. So a 2 tile island that only has a fully developed campus that somehow reaches say, 16 pop (sea resources) is going to generate as much science as a 36 tile city that has a fully developed campus (with no mountains) and whatever else it's done with the excess tiles, provided both cities are 16 pop.
The difference is that the 36 tile, 16 pop city will likely be adding other yields (gold, campus, wonder bonuses, etc) to your empire and your tiny island city would be hyper-specialized to producing only the yield of the one district.